Are ES Modules garbage collected? If so, do they re-execute on next import?

#!/JoePea joe at
Wed Jul 15 05:16:10 UTC 2020

How can we ensure that long-running applications (even if theoretical),
that may load and unload an unlimited number of new modules over time
(f.e. routes in a web page specified by 3rd parties as time
progresses), not leak memory?

Even if it is theoretical, I don't like the thought of something that
only ever allocates memory that will never be freed.

Is someone working on a solution for this?


On Wed, Jul 1, 2020 at 6:16 AM Mark S. Miller <erights at> wrote:
> No, definitely not. The table from specifiers to module instances is indexed by specifiers. Specifiers are strings, so this table is not weak. It is not a "cache" in the sense that it is allowed to drop things. Rather it is a registry of module instances. Only a registry as a whole can be gc'ed, and which point that context is no longer around for instantiating or reinstantiating modules.
> As you suggest, if it could drop things because of GC that it would then need to regenerate, that would expose the non-determinism of gc. That would be a big deal. We carefully designed WeakMaps so that gc was non-observable. WeakMaps introduce no observable non-determinism. WeakRefs alone expose the non-determinism of gc, and are kept well quarantined from the rest of the language.
> On Tue, Jun 30, 2020 at 5:42 PM #!/JoePea <joe at> wrote:
>> I am curious: can modules be garbage collected if the exports are not
>> references by anything anymore? And if so, will the module be
>> re-evaluated the next time it is imported?
>> I haven't tried an experiment to answer this yet. I'll be back to post
>> findings if someone doesn't post an official answer first.
>> I'm thinking about code longevity. For example, if we make
>> long-running web-based applications with many routes and features (for
>> sake of example imagine a desktop environment, or a MMORPG game, with
>> apps or components that are loaded within the same context). Over
>> time, if imports are not collected, then it means we have a memory
>> leak.
>> Imagine, for example, an infinite-universe MMORPG where you can land
>> on different planets where the code for features of a planet are
>> provided by third parties as ES Modules. I know, this might not be a
>> safe idea to import any code into an app, but just imagine it for sake
>> of example (imagine we have a continuous integration system to test
>> and verify code security, or something, before that code is allowed to
>> be consumed in the app). Imagine you play this app for many many days,
>> and visit many places, and you leave the app running the whole time
>> (because farming for resources is disabled if the app is not running,
>> or something).
>> I would imagine that we want unused modules (when we leave a planet,
>> for example) to be (destroyed) garbage collected so that we don't
>> waste memory.
>> #!/JoePea
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> --
>   Cheers,
>   --MarkM

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